The invention relates to windows for conveyances of all kinds including aircraft, ships and land vehicles, and is especially applicable to windows for use in conveyances having military application.
Although many sophisticated plastics are now available, glass still possesses important advantages for use in windows, at least as a component or ply of a laminated window. Apart from its durability, it is more rigid than most available plastics and is not adversely affected by the application or use of an electrically conductive heating film on its surface. It is thus commonly used as the outer ply of even the most sophisticated windows.
A disadvantage of using glass as the outer ply, especially in military applications, is its pronounced reflections at two regions in the far infra red spectrum, namely 8-12 microns and 18-25 microns. This is a known phenomenon and the reflection peaks are known as "Restrahlen Bands". These reflections, especially the reflection of incident radiation from the glass at a wavelength of 8-12 microns, enable a vehicle equipped with a window with a glass outer ply to be detected by infra red instruments and indeed, when the window is fitted to a helicopter, results in a highly characteristic intermittent infra red reflection which can be detected and readily recognized by opposing military forces. It would therefore be desirable, for military application, to suppress the reflectivity of glass outer plies in the far infra red region of the spectrum.
In the past, efforts have been made to suppress infra red reflections of glass in order to increase the emissivity of glass surfaces on solar cells by using a multilayer thin film tailored to reduce such reflections. However, such efforts have resulted in only modest reductions in the Restrahlen Band reflections as illustrated in the graph forming FIG. 5 of the accompanying drawings and discussed later in this specification.
Moreover, besides the modest improvement achieved, this obvious approach to the problem has a number of serious disadvantages:
(a) because the films are necessarily thin (typically of the order of 50 to 500 nm), but the thickness of the layers is critical, their performance, at least in airborne applications, inevitably deteriorates as a result of abrasion. PA1 (b) abrasion is also liable to result in a deterioration of optical performance of the coating, in particular the onset of haze and consequent reduction in resolution. PA1 (c) it would be technically difficult, and consequently expensive, to deposit the required complex multilayer films with the required degree of precision over large pieces of curved glass.
The present invention aims to overcome, or at least alleviate, the above disadvantages and is based on a completely different approach to the problem.